docker-zonefiled

What's this?

This generates DNS zone files containing DNS hostnames and aliases of running docker containers in all docker networks for a given docker host.

It generates zone files like this:

container_name.network_name.in-docker.internal. A 172.25.1.2
alias.network_name.in-docker.internal. AAAA 2a02:F00:BA4::1:2
container_name.backend_network.in-docker.internal. A 192.168.42.2

This is currently implemented as an trivial simple bash script utilizing docker events for listening on changes, docker container inspect to query the network details and some jq magic.

Why 'tho?

• I wanted sane dns resolution from the docker host to docker containers.
• nss-docker isn't enough or working.
• DNS operations are well understood. Automagic name resolution via nss leads to unexpected behavior, if container name vanished and queries drop down to dns.
• Using DNS subdomains host and aliases are structured per network name.
• Your networking, routing and firewalling decides if you're able to reach the container interfaces.
• Passing a different docker socket and alternative domain zone name can support multi-docker-setups.

Remarks

Input sanitization of user controlled values

Specially grafted host and alias names of docker containers can inject stuff in the generated zone file, I guess. I'm currently open to discuss, how to catch invalid characters in host names.

Currently used .internal TLD

This is currently used since it should never be resolved in the internet. Since it could be used in a private / enterprise context, it could be conflicting existing setups. I'm very open to discuss if a .in-docker TLD would be a sane idea.